1. Field of the Invention
The present invention relates to a display device including a multi-grayscale processing circuit for subjecting an input video signal to a multi-grayscale process.
2. Description of the Related Art
In recent years, as a two-dimensional image display panel, a plasma display panel (hereinafter, referred to as PDP) having a plurality of discharge cells arranged in matrix has been receiving attention. For displaying any image corresponding to an input video signal on such a PDP, a subfield method is known as a driving method. With the subfield method, the display period of a field is divided into a plurality of subfields, and on the resulting subfield basis, the discharge cells are each selectively discharged for light emission depending on the luminance level of the input video signal. This allows perception of intermediate luminance corresponding to the total duration of light emission in a field period.
FIG. 1 is a diagram showing an exemplary light emission driving sequence based on such a subfield method (refer to FIG. 14 of Japanese Patent Kokai No.2000-227778 (Patent Document 1) as an example).
In the light emission driving sequence of FIG. 1, a field period is divided into fourteen subfields of SF1 to SF14. Only in the subfield SF1 locating first of those SF1 to SF14, all of the discharge cells of a PDP are initiated to be in a lighting mode (Rc). On the basis of each subfield SF1 to SF14, an input video signal is referred to set the corresponding discharge cells to an extinction mode (Wc), and only the discharge cells in the lighting mode are discharged for light emission for the duration allocated to the subfield (Ic).
FIG. 2 is a diagram showing an exemplary light emission driving pattern in a subfield period of each discharge cells to be driven based on such an light emission driving sequence (refer to FIG. 27 of Patent Document 1, for example).
In the light emission.pattern of FIG. 2, the discharge cells initiated to be in the lighting mode in the first subfield SF1 are set to the extinction mode as shown by black dots in any one of the subfields SF1 to Sf14. Once set as such, those are not put back to the lighting mode again. Thus, the discharge cells continuously discharge for light emission in the subfields as shown by white dots until set to the extinction mode. At this time, the fifteen light emission patterns of FIG. 2 vary in total light emission duration in a field period, representing fifteen intermediate luminance levels. That is, achieved thereby is intermediate luminance display of (N+1) tones (where N is the number of subfields).
The problem with such a driving method is that the subfields as a result of field division are limited in number, causing shortage of the number of tones. Thus, to make up for the tone shortage, the input video signal is subjected to a multi-grayscale process such as error diffusion and dithering.
First, in the error diffusion process, an input video signal is converted into pixel data on a pixel basis, for example pixel data of eight bits. Out of the resulting data, six significant bits are regarded as display data, and the remaining two less-significant bits as error data. Then, the error data of the pixel data derived for each pixel in a close range is assigned weights and added together, and the result derived thereby is reflected to the display data. Through such an operation, as to one original pixel, the luminance of the less-significant two bits is represented in a pseudo manner by other pixels therearound, enabling representation of luminance tone equivalent to pixel data of eight bits using display data of only six bits. Then, the error-diffused pixel data of six bits derived by such an error diffusion process is subjected to dithering. At dithering, a plurality of adjacent pixels are regarded as a pixel unit, and to the error-diffused pixel data corresponding to each pixel in the pixel unit, a dither coefficient is assigned. The dither coefficients vary in value, and after such assignment, the dither coefficients are added together. Through such addition of dither coefficients, in view of a pixel unit, luminance representation so far required eight bits can be achieved only by four significant bits of the dither-added pixel data. Accordingly, four significant bits of the dither-added pixel data are extracted, and the extraction result is assigned to 15 light emission patterns of FIG. 2 as multi-grayscale pixel data PDs.
Here, another problem of image quality degradation arises if addition of dither coefficient to pixel data is done regularly by dithering, for example. This is because pseudo patterns irrelevant to an input video signal, so-called dither patterns, may be perceived thereby.
The present invention is proposed for solving the above problems, and an object thereof is to provide a display panel drive capable of satisfactory image display with dither patterns suppressed.